It is well known that there is an ideal temperature range and degree of plasticization for peeling veneer from logs, blocks or bolts, (generically "logs") of various wood species, varying from room temperature, to about 200.degree. F., and various methods have been proposed and are in use for conditioning the logs. The conditioning process being used can itself introduce problems that will affect the peeling operation and the quality of the veneer produced.
Because of the nature of the cellular structure of wood, heating and cooling proceed about two and one half times faster in the longitudinal direction from the ends towards the middle than in the radial direction from the outer diameter towards the core. Thus, too rapid heating if unaccompanied by sufficient tempering can result in logs having some portions that are too highly plasticized and other portions that are insufficiently conditioned. Slow conditioning can increase end splits and discolor the wood. Nosebar adjustments at the peeling lathe can accommodate some differences in degree of plasticization but in general, if in spite of these adjustments the veneer is loosely cut a higher log temperature or longer heating period is indicated and if there is fuzzy cutting about the whole circumference of the log a lower log temperature or shorter heating period is indicated, as the log has been over-softened.
Although some plywood mills run the same species, diameter, quality and degree of freshness of logs for weeks on end, it is more typical that a mill, over time, will face an input of various species and logs of varying diameter, quality and length of time since harvesting, for which the ideal conditioning parameters will correspondingly vary.
The present inventor is most acquainted with the practices at Southern plywood mills in the United States and came to consider the need for the present invention in connection with surveying the energy requirements of one or more of such mills in the course of studying ways to reduce energy input requirements as existing plants are kept in adequate repair and modernized. Of course the principles of the invention are equally applicable to mills being newly constructed.
In the experience of the present inventor, practically every Southern plywood mill uses some means for heating logs prior to peeling veneer from them while at elevated temperature. And of the log conditioning processes currently in commercial use, it appears that most often, the conduct of the heating process, whether it be by steam, or live steam mixed with water, or steam and dryer condensate, or steam and press condensate and boiler blowdown, little attention is given to maintenance and proper conduct during long periods of fairly continuous periods of operation between down times for maintenance and repair. In a way, the log heating operation is the orphan child of the typical mill. While the operators continue to go through the motions of block preparation, as deterioration sets in and debris accumulates, the conditioning is less and less ideally performed but typically no one does anything about it until the point of near total failure is reached, when the capital outlay required for refurbishing or replacement of the deteriorated log conditioning system is staggering, and may not be available.
Most of the Southern mills use simple soaking vats in which the logs are stepped in heated liquid. Others use shedded-over drive-in vaults, (or less typically) treatment tunnels with a series of interior chambers partially segregated from one another by hanging curtains.
Fifty percent or more of the mills in the present inventor's experience move their blocks through a liquid in soaking vats. These vats have problems of flow interruption and jamups, which can be coped with by adjusting the liquid level control and by having either crane-type hoists or other hoists handy to pull blocks in case of jamups or damage to the conveying equipment. If the soaking vat is heated by coils immersed in the bottom of the vat or if live steam is injected through drilled lines, it is, of course, necessary to keep these free from debris and trash. Often the trash and debris is not removed with sufficient frequency to insure that the maximum heat penetration, and thus the maximum yield potential, is being achieved. So what one sees when one inspects the vats is an accumulation of debris over the lines in the bottom of the vat through which the water or steam must pass, and water lines in the center of the floor covered by debris. Conditions vary from mill to mill. One line may be covered with debris and another may be open. Steam lines can br torn or plugged or have broken nozzles, vat doors may be missing, hanging curtains torn; dry kiln doors may have been ruined by fork-lift operators. The result of all this is that blocks may be hotter on the top tier than on the bottom tier, blocks may be progressively cooler from end to end, blocks may vary from one vat to another in degree of conditioning and blocks may be hotter at the back of the vat than at the front. Under these conditions, one loses yield and creates problems that are carried all the way through the mill, in long drying schedules, in redry, in long pressing schedules, and in a very low percentage when it comes to shear tests, in increased clipper loss, in drying downgrade and in higher reclip. All these problems are aggravated by poor housekeeping.
One characteristic of soaking vats is the high acidity that comes about by the continuous reuse of the soaking liquid. Testing of these vats has shown pH as low as 3.5. This limits yield, because it increases the brittleness of the fiber, resulting in breaking of the fiber and deeper lathe checks than would have been the case if the blocks had not moved through this acid bath. In addition, the acid contributes to dissolving of the steam line, dissolving of the conveyor linkages, and dissolving of the lathe chucks.
In its natural state wood is acidic. That is what contributes to the strength and brittleness of the fibers. The average composition of wood is 25% lignin, 25% hemicelluloses and 50% cellulose; lignin and hemicelluloses are thermoplastic, that is, they are softened by heat. So when the block is subjected to heat, 50% of the block is naturally softened. The other 50%, the cellulose portion, is acid and is not softened by heat. It remains brittle, breaks rather than cuts, and this contributes to deep lathe checks.
It is known that in an alkaline environment, the cellulose comprising the other approximately 50% of the typical log also becomes plasticized during conditioning. An alkaline environment, say with a pH of 9, for example, softens the cellulose so that it can be cut rather than broken and reduces the depth of lathe checks. Since the blocks are acidic, they soak up this alkaline solution just like a sponge. The alkali saponifies the impacted resins so they wash out of the block. The penetration of heat is more uniform, so in rotary peeling a solid ribbon can be peeled all the way to the core, with the exception of tipple breaks and jamups. Using an alkaline solution means that one can cut a full ribbon all the way down to a 4 inch core. It means the D full sheets stay intact all the way down the line through the dryer. It means uniform heat penetration end-to-end throughout the length of the block. It means a narrow moisture band and no breaks as the peel enters a knot area. It means the knots will not crumple and fall out and ring knots will not break. Thus this type of treatment can reduce clipper loss, reduce dryer falldown, reduce redry, reduce reclip and reduce waste. Complete block plasticization has much to offer in terms of increased yield, over the best that can be achieved with existing processes which are carried out under acidic conditions. To compare a steam and water system with wet alkaline steam at pH of 9, tests were run on southern pine veneer blocks under equal and controlled mill conditions. The vats were cleaned and in good repair. The tests were run in the summer so the results are considered to be conservative for year-around use. Panel yield from a wet alkaline process exceeded yield from the steam and water process by 7 percent. To determine dollar value, cost analysis of southern pine industry figures for 1973 were used. The cost of wood alone was reduced $2.73 per thousand square feet. If viewed as production increase and green end labor is distributed over the increased volume, it is greater and if one uses the yield increase as increased production and distribute it over fixed costs and overhead in addition to green-end labor, the return is at maximum.
The energy losses encountered in conducting typical prior art log conditioning processes leave much room for improvement. Plywood and veneer mills surveyed by the present inventor which use soaking vats acknowledge a loss of about 50 percent of B.T.U. input due to evaporation and other heat transfer to outside the system; for those mills using vaults, the reported B.T.U. loss is 30-50 percent. This loss runs from $82,000.00 to $108,000.00 per year for every five vats in use, at present fuel prices. So few mills use the tunnel conditioning of Gates et al., U.S. Pat. No. 3,750,303, issued Aug. 7, 1973, that the present inventor has been unable to find reported B.T.U. loss statistics covering use of that system.
Problems with maintenance of prior art log conditioning systems include the following:
A. VAT-TYPE SYSTEM PA0 B. VAULT-TYPE SYSTEM PA0 C. TUNNEL-TYPE SYSTEM
1. Debris clogs the steam and/or hot water inlets. PA1 2. Logs break heating pipes. PA1 3. The conveyor chains break. PA1 4. The conveyor chains lock-up due to wedging-in of trash, and trash accummulation. PA1 5. High acidity corrodes metal parts exposed to vat liquid carried downstream in process with the conditioned logs. PA1 1. Vault doors are ruined by being roughly opened and closed by using fork-lifts, causing excessive leakage. PA1 2. Heating lines are broken by logs. PA1 3. Steam nozzles are broken by logs or by fork-lift operators. PA1 4. Nozzles without filters become plugged. PA1 1. Log jamups tear baffles. PA1 2. Nozzles and lines are broken by impact of cross logs and log jams. PA1 3. Log chains jam and break. PA1 4. There are many parts which are easily damaged or which wear-out before a general overhaul would otherwise be needed.